从每日水果价格表开始
fruits.csv
Day,Name,Kind,Price
2019-09-04,"apple","red",63.09
2019-09-04,"apple","yellow",52.14
2019-09-04,"orange","navel",41.18
2019-09-04,"orange","blood",41.18
2019-09-03,"apple","red",63.07
2019-09-03,"apple","yellow",52.11
2019-09-03,"orange","navel",41.13
2019-09-03,"orange","blood",41.13
我想按名称和种类插入参考价格
fruit_ref_prices.csv
Name,Kind,Reference_Price
"apple","red",60.00
"apple","yellow",50.00
"orange","navel",40.00
"orange","blood",42.00
得出下表
Day,Name,Kind,Price,Reference_Price
2019-09-04,"apple","red",63.09,60.00
2019-09-04,"apple","yellow",52.14,50.00
2019-09-04,"orange","navel",41.18,40.00
2019-09-04,"orange","blood",41.18,42.00
2019-09-03,"apple","red",63.07,60.00
2019-09-03,"apple","yellow",52.11,50.00
2019-09-03,"orange","navel",41.13,40.00
2019-09-03,"orange","blood",41.13,42.00
使用C＃内置的类似SQL的语法，该解决方案应该很简单，我相信答案就在以下教程页面之一中：
但是我很难识别这种语言的语法。
我在下面尝试而不是写
join fruit_ref in fruit_refs on fruit.name equals fruit_ref.name
我应该会写
join fruit_ref in fruit_refs on fruit.name equals fruit_ref.name
and fruit.kind equals fruit_ref.kind
但是不接受布尔表达式。为什么？
我的尝试是：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using System.IO;
namespace MyConsoleApplication
{
class Program
{
const string root = @"c:\path\to\here\";
const string file1_in = root + @"fruits.csv";
const string file2_in = root + @"fruit_ref_prices.csv";
static void Main(string[] args)
{
Fruit_Basket fruit_basket = new Fruit_Basket(file1_in, file2_in);
fruit_basket.PrintFruits();
}
}
public class Fruit
{
public DateTime day { get; set; }
public string name { get; set; }
public string kind { get; set; }
public decimal price { get; set; }
public Fruit(DateTime newFruit_day,
string newFruit_name,
string newFruit_kind,
decimal newFruit_price)
{
this.day = newFruit_day;
this.name = newFruit_name;
this.kind = newFruit_kind;
this.price = newFruit_price;
}
}
public class Fruit_Ref
{
public string name;
public string kind;
public decimal reference_price;
public Fruit_Ref(string newName, string newKind, decimal newRef_Price)
{
this.name = newName;
this.kind = newKind;
this.reference_price = newRef_Price;
}
}
public class Fruit_Basket {
public List<Fruit> fruits { get; set; }
public List<Fruit_Ref> fruit_refs { get; set; }
public Fruit_Basket(string file1_in, string file2_in) {
build_fruit_list(file1_in);
build_fruit_ref_list(file2_in);
}
public void build_fruit_list(string file_in)
{
fruits = new List<Fruit>();
int count = 0;
StreamReader reader = new StreamReader(file_in);
string line = "";
while ((line = reader.ReadLine()) != null)
{
if (++count > 1)
{
string[] splitLine = line.Split(new char[] { ',' }).ToArray();
var newFruit_day = DateTime.Parse(splitLine[0]);
var newFruit_name = splitLine[1];
var newFruit_kind = splitLine[2];
var newFruit_price = decimal.Parse(splitLine[3]);
Fruit newFruit = new Fruit(newFruit_day,
newFruit_name,
newFruit_kind,
newFruit_price);
fruits.Add(newFruit);
}
}
reader.Close();
}
public void build_fruit_ref_list(string file_in)
{
fruit_refs = new List<Fruit_Ref>();
int count = 0;
StreamReader reader = new StreamReader(file_in);
string line = "";
while ((line = reader.ReadLine()) != null)
{
if (++count > 1)
{
string[] splitLine = line.Split(new char[] { ',' }).ToArray();
var newFruit_name = splitLine[0];
var newFruit_kind = splitLine[1];
var newFruit_ref_price = decimal.Parse(splitLine[2]);
Fruit_Ref newFruit_ref = new Fruit_Ref(newFruit_name,
newFruit_kind,
newFruit_ref_price);
fruit_refs.Add(newFruit_ref);
}
}
reader.Close();
}
public void PrintFruits()
{
var innerJoinQuery =
from fruit in fruits
join fruit_ref in fruit_refs on fruit.name equals fruit_ref.name
select new { Day = fruit.day, Name = fruit.name, Kind = fruit.kind,
Price = fruit.price, Reference_Price = fruit_ref.reference_price };
Console.WriteLine($@"""Date"",""Name"",""Kind"",""Price"",""Ref Price""");
foreach (var i in innerJoinQuery)
{
Console.WriteLine($@"{i.Day},{i.Kind},{i.Price},{i.Reference_Price}");
}
}
}
}
[请将equals子句更改为on new { fruit.name, fruit.kind } equals new { fruit_ref.name, fruit_ref.kind }
为什么需要这个
查询具有两种匿名类型（一种用于左表，一种用于右表）。因此，要比较那些匿名类型和比较它们，该语句应使用new关键字
查询：
var innerJoinQuery = from fruit in fruits
join fruit_ref in fruit_refs on new { fruit.name, fruit.kind } equals new { fruit_ref.name, fruit_ref.kind }
select new { Day = fruit.day, Name = fruit.name, Kind = fruit.kind,
Price = fruit.price, Reference_Price = fruit_ref.reference_price };
您还可以重构代码以使用CsvHelper NuGet包读取/写入CSV文件。
首先，您可以使这些类反映水果，水果参考和最终的水果结构。
public class Fruit
{
public string Day { get; set; }
public string Name { get; set; }
public string Kind { get; set; }
public string Price { get; set; }
}
public class FruitReferencePrice
{
public string Name { get; set; }
public string Kind { get; set; }
public string Reference_Price { get; set; }
}
public class FruitFinal
{
public string Day { get; set; }
public string Name { get; set; }
public string Kind { get; set; }
public string Price { get; set; }
public string ReferencePrice { get; set; }
public override string ToString()
{
return $"Day={Day},Name={Name},Kind={Kind},Price={Price},Reference_Price={ReferencePrice}";
}
}
然后，您可以使用两种方法将每个CSV文件的行返回到List<Fruit>和`List。
private static IEnumerable<Fruit> BuildFruitList(string csvFilePath)
{
if (!File.Exists(csvFilePath))
{
throw new FileNotFoundException("Could not locate CSV at path " + csvFilePath, csvFilePath);
}
try
{
using var fileReader = File.OpenText(csvFilePath);
using var csv = new CsvReader(fileReader);
return csv.GetRecords<Fruit>().ToList();
} catch (Exception ex)
{
Console.WriteLine(ex.Message);
return Enumerable.Empty<Fruit>().ToList();
}
}
private static IEnumerable<FruitReferencePrice> BuildFruitReferenceList(string csvFilePath)
{
if (!File.Exists(csvFilePath))
{
throw new FileNotFoundException("Could not locate CSV at path " + csvFilePath, csvFilePath);
}
try
{
using var fileReader = File.OpenText(csvFilePath);
using var csv = new CsvReader(fileReader);
return csv.GetRecords<FruitReferencePrice>().ToList();
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
return Enumerable.Empty<FruitReferencePrice>().ToList();
}
}
然后您可以执行分组联接并输出合并的结果。
var path1 = "PATH\\fruits.csv";
var path2 = "PATH\\fruit_ref_prices.csv";
var fruitList = BuildFruitList(path1);
var fruitReferencePrices = BuildFruitReferenceList(path2);
var groupedJoin = from fruit in fruitList
join fruit_ref in fruitReferencePrices
on new { fruit.Name, fruit.Kind } equals new { fruit_ref.Name, fruit_ref.Kind }
select new FruitFinal
{
Day = fruit.Day,
Name = fruit.Name,
Kind = fruit.Kind,
Price = fruit.Price,
ReferencePrice = fruit_ref.Reference_Price
};
foreach (var fruit in groupedJoin)
{
Console.WriteLine(fruit.ToString());
}
合并结果：
Day=2019-09-04,Name=apple,Kind=red,Price=63.09,Reference_Price=60.00
Day=2019-09-04,Name=apple,Kind=yellow,Price=52.14,Reference_Price=50.00
Day=2019-09-04,Name=orange,Kind=navel,Price=41.18,Reference_Price=40.00
Day=2019-09-04,Name=orange,Kind=blood,Price=41.18,Reference_Price=42.00
Day=2019-09-03,Name=apple,Kind=red,Price=63.07,Reference_Price=60.00
Day=2019-09-03,Name=apple,Kind=yellow,Price=52.11,Reference_Price=50.00
Day=2019-09-03,Name=orange,Kind=navel,Price=41.13,Reference_Price=40.00
Day=2019-09-03,Name=orange,Kind=blood,Price=41.13,Reference_Price=42.00